The present invention relates to amplifier circuits. More particularly, the present invention relates to a base current compensation circuit for use in an amplifier circuit.
The increasing demand for higher performance amplifier circuits has resulted in the continued improvement of the precision and accuracy of the various devices and components within the amplifier circuits, as well the inclusion of additional buffers and compensation circuits.
In the implementation of various amplifier circuits, losses in the integrity of referenced currents flowing through the various devices and components can be realized. As a result, the delivered current can be less than the intended current to be provided from the amplifier circuit. One problem that can cause losses in the integrity results from the absorption of base current that is demanded from transistor devices by other devices within the amplifier circuit.
For example, with reference to FIG. 1, a circuit 100 comprising a differential pair of transistors Q1 and Q2, such as may be used within a logarithmic amplifier circuit, are illustrated. A first reference current IC1 is provided to the collector of transistor Q1, while a second reference current IC2 is provided to the collector of transistor Q2. It is desirable for a base current IB1 demanded by transistor Q1 to be provided without loss, such that a lost current ILOST equals zero. However, prior art amplifier circuits unfortunately are not able to provide the base current without loss. As a result, lost current ILOST can also generate error voltages that create an output error for amplifier circuit 100.
One approach for compensating for lost current ILOST includes the use of an isolating buffer that creates an additional current for supplementing the lost current ILOST and that is provided at the base of the transistor Q1. However, such isolating buffer applications are generally more complex than desired. With reference to FIG. 2, another approach includes the implementation of a current mirror comprising diode-connected transistor Q3 and transistor Q4 configured to approximately provide the base current IB2 from the base of transistor Q2 to the base of transistor Q1, wherein transistors Q1 and Q2 are matched, and transistors Q3 and Q4 are matched. However, this approach requires significant headroom since the input current into the collector of transistor Q2 requires the voltage at the collector of transistor Q2 to be at least a base-emitter voltage drop VBE above the voltage at the base of transistor Q1. For many amplifier circuits, such as logarithmic amplifier circuits, it is desirable for the voltage at the collector of transistor Q2 be equal to the voltage at the base of transistor Q1.
Accordingly, a need exists for addressing the lost base current resulting within amplifier circuits.
In accordance with various aspects of the present invention, a base current compensation circuit is configured for injecting base current to the base of a transistor device to compensate for the lost current demanded by a transistor base. The base current compensation circuit is configured to inject current into the base of the transistor without the headroom requirements, as well as being less complex than other approaches.
In accordance with an exemplary embodiment, a base current compensation circuit comprises a sampling circuit configured for sampling current from the transistor device, and for providing multiples of the base current demanded by the transistor device. The sampling circuit can comprise a first sampling component for sampling current comprising a collector current proportional to the collector current of the transistor device. The sampled collector current can be configured through a second sampling component into a sampled base current proportional to the base current demanded by the transistor device. The sampled base current can then be provided to the base of the transistor device to compensate for current lost at the base. Base current compensation circuit can also comprise a current mirror circuit coupled between the first sampling component and the second sampling component for facilitating the sampling functions.